Poly(tetramethylene ether) glycol (PTMEG) is produced by polymerizing tetrahydrofuran (THF) in presence of an anhydride, typically acetic anhydride (ACAN), to form the diacetate of the polymerized tetrahydrofuran (PTMEA), which must be converted to the corresponding dihydroxy product (PTMEG).
As extensively described in the technical literature, the acetate end groups are removed by submitting PTMEA to transesterification with an alcohol, usually methanol, in presence of an alkaline catalyst. After the transesterification is complete, the alkaline catalyst is removed by reacting the catalyst with an acid forming a salt which is removed by filtration. The technology of transesterification is well known and described in the technical literature, as for instance in U.S. Pat. No. 2,499,725, Journal of Americal Society Vol. 70 pg. 1842, and in Groggings “Unit processes in organic synthesis” pg. 710-715.
Since the presence of PTMEA in the PTMEG product, even in small amount, is not desirable in urethane applications, high level of conversion, typically above 99,9%, is to be achieved in the transesterification. Another critical factor affecting the quality of the product is the presence of alkalinity or of acidity in the product.
The operation in batch of the transesterification and of the neutralization permits a positive control of the performances in order to assure the production of PTMEG meeting the critical standards of quality.
The operation in batch, however, requires high and discontinuous consumptions of steam, bigger equipment, increased manpower.
A continuous mode of operation is by all means desirable, but, compared to the batch mode, has a lower degree of flexibility and consequently has a higher risk of yielding off spec PTMEG.
Consequently in the design of a continuous transesterification system special provisions have to be taken to assure high level of performances within the wide range of operating conditions encountered in the practice of industrial plants.
Such special provisions do not appear enough defined in the previous art.
In WO 97/23559 DU PONT describes a process of reactive distillation where the feed to the alkanolysis, consisting of PTMEA, an alkanol and an alkaline catalyst is fed to the top section of a distillation column while hot vapors of alkanol are fed to the bottom of the column to sweep any alkanol ester formed by alkanolysis of PTMEA.
In the above mentioned patent DU PONT states that the processes described in his previous U.S. Pat. No. 4,230,892 and U.S. Pat. No. 4,584,414 fall to assure such high level of performances are required in a process operating in a continuous mode of operation.
However, the statement in WO 97/23559 that the alcoholysis by reactive distillation may be performed by means of any of the distillation methods and equipment as generally known and practiced in the art, including conventional tray distillation columns, does not appears to provide by all means satisfactory results. In fact incomplete conversions have been achieved when performing alcoholysis in a conventional column with multiple trays, in case the global retention time of the liquid reagents in the column is fairly low, for instance less than half an hour.
As clearly described in Grogging “Unit processes in organic synthesis” chapt. 12, the correct design of a continuous multistage transesterification reactor requires the knowledge not only of the thermodynamics but also of the laws of mass action and of kinetics, which determine the speed of the reaction, the retention time necessary, and consequently the approach to equilibrium in each stage.
In case of inadequate retention time the reaction does not evolve to equilibrium, and the performances differ substantially from the values predictable by the conventional methods of distillation.
About the neutralization, the previous art does not offer fully satisfactory solutions in relation to a continuous mode of operation.
U.S. Pat. No. 4,460,796 describes a process where the alkaline catalyst is removed by reacting with orthophosphoric acid in the equivalence ratio of from 1.5 to 2.5:3. The salt formed is subsequently separated from PTMEG by filtration.
The process appears in principle adequate in case the neutralization is performed batchwise, but is not free of risks in a continuous mode of operation, where it may result not easy to control the addition of reagent acid within the specified range of equivalents, in consideration of the fluctuations which occur in the industrial practice.
In case of abnormal operation the PTMEG product will be contaminated either by an abnormal content of alkali or by an excess of acid.